1. Field of the Invention
The invention relates to a cooling device comprising Peltier elements for a thermally highly loaded optical crystal, or laser crystal, respectively, from which laser beams, in particular laser pulses, are obtained, e.g. for the laser crystal of an optical amplifier or oscillator.
2. Description of Related Art
An effective cooling of optical crystals, or laser crystals, respectively, xe2x80x9ccrystalsxe2x80x9d in short hereinafter, in laser devices is of particular importance if the crystals, e.g. titanium-sapphire crystals (commonly termed Ti:S laser crystals) are subjected to high thermal loads during operation. This is, e.g., the case if in a passively mode-locked short-pulse laser arrangement (oscillator) the crystal is utilized as an optically non-linear element, and the pump beam and the resonator beam are focussed as highly as possible in the crystal; in doing so, the crystal should have small dimensions and, for compensation thereof, a high dotation so as to keep low the material dispersion, whereby thexe2x80x94specificxe2x80x94thermal load will rise, as has been explained in the earlier application WO-98/10 494-A not previously published. There it has also been explained that cooling to below 10xc2x0 C. is a problem because of the humidity condensation occurring in that instance, wherein little drops condensed on the crystal may cause the crystal to be damaged rapidly or even to be destroyed.
What is of quite particular importance is, moreover, cooling of the crystal in case of an optical amplifier, as has already been mentioned in Optics Letters Vol. 22, No. 16, Aug. 15, 1997, pp. 1256-1258, xe2x80x9c0.2-TW laser system at 1 kHzxe2x80x9d by Backus et al. In such an optical post-amplification of oscillator pulses, e.g., also a Ti:S laser crystal is used in which the pulses from the oscillator having an energy of some nJ are amplified to an energy in the order of 1 mJ (i.e., by the factor 106). To this end, the Ti:S amplifier crystal is xe2x80x9cpumpedxe2x80x9d with green laser light which, e.g., has an average power of 10 to 20 W, which is a multiple of the pumping power at the laser pulse generation in the oscillator. Also by the fact that the optical amplifier is operated in pulses (the pulse frequency being, e.g., approximately 1 kHz), the pumping energy is concentrated to individual pulses which amplify the oscillator pulses. Due to the high powers occurring there, it is important to attain sufficient cooling for the crystal. Insufficient cooling of the crystal will not only result in a poor efficiency, similarly as with the oscillator, but also in an unfavorable beam profile, due to the xe2x80x9cthermal lensesxe2x80x9d effect which also is explained in the afore-mentioned article by Backus et al. If the crystal is heated, the temperature gradient thus occurring in its material will lead to a refraction index gradient which will unintentionally focus or defocus the laser beam during its passagexe2x80x94depending on the crystal material. Good cooling of the crystal will increase the thermic conductivity of the crystal material, and the temperature coefficient of the refraction index (which causes the xe2x80x9cthermal lensexe2x80x9d effect) becomes smaller at the low temperatures so that a beam profile approximately corresponding to the ideal Gaussian intensity profile (over the cross-section) will be attained; moreover, the degree of efficacy will be improved. According to the article by Backus et al., liquid nitrogen is used to cool the crystal, which does make it possible to attain extraordinarily low temperatures, by which, however, a practicable embodiment of the optical amplifier is prevented for many purposes of application, in particular for mobile uses.
A somewhat different optical amplifier has been described in the article xe2x80x9cGeneration of 0.1-TW 5-fs optical pulses at a 1-kHz repetition ratexe2x80x9d by S. Sartania et al., Optics letters Vol. 22, No. 20, Oct. 15, 1997, wherein general mention is made that a Peltier cooling device is used for cooling the amplifying crystal. Thus, the problem remains that with an intensive cooling not only condensation water will form on the crystal, but even ice, and that contaminations are present in the air which will deposit on the crystal; in operation, such ice formations and contaminations will lead to axe2x80x94localized xe2x80x94destruction of the crystal surface by burning in.
It is an object of the invention to overcome these problems and to provide a cooling device of the initially defined type with which, on the one hand, in spite of a simple construction that will render it particularly suitable for mobile applications, a good cooling in terms of a high degree of efficacy and an optimum beam profile will be achieved, and by which, on the other other hand, a long useful life of the laser crystal will be ensured by avoiding burning in of condensation water (ice), or impurities, respectively.
The inventive cooling device of the initially defined type is characterized in that the crystal, together with the Peltier elements provided for its cooling, is housed in an encasing container, that the interior of the container is evacuated and/or kept dry by means of a desiccating substance, and that the container comprises at least one Brewster window for the passage of the laser beams which is arranged under an angle relative to the optical axis which corresponds to the Brewster angle.
By providing an encasing container it becomes possible to evacuate the container interior or to keep it dry so that condensation water cannot deposit on the optical crystal, or laser crystal, respectively; moreover, defined clean surroundings (vacuum or pure, i.e. contamination-free, dry air) are possible for the crystal. Accordingly, long operating times can be achieved which is a great advantage also with a view to the expenditures required during the installation or during the precise adjustment of optical crystals, or laser crystals, respectively. Moreover, the present cooling device is characterized in that as a consequence of the use of the thermoelectric cooling elements, i.e. Peltier elements, in combination with the encasing container, a compact, simple, handy construction of the laser arrangement is made possible whereby, moreover, its use in vehicles, e.g. also in airplanes, is possible without any problems, since in contrast to cooling with liquid nitrogen, it is not gravity-dependent during its operation. The container may be provided with a tightly closable connection means for an evacuation as well as with tightly sealed electrical line passages for the power supply of the Peltier elements.
With a view to the high intensities occurring in the applications in question, so-called Brewster windows are provided on the container for the passage of the laser beams. In this manner, unintentional reflections can be prevented, i.e. without the broad-band antireflex coatings otherwise used therefor; because such dielectric coatings would not withstand the afore-mentioned high intensities (e.g. peak powers in the MW to GW range at beam diameters of  less than 10 mm and at pulse durations in the 10 fs to ps range, starting from an average power of 10 mW up to the watt range; pump parameters: average power, a few W up to a few 10 W; pump energy, a few mJ; high repetition frequencies in the kHz range which will lead to peak powers in the kM to MW range).
It should be mentioned that with semi-conductor lasers it is known to use encapsulated modules, cf., eg., DE 33 07 933 C, DE 39 22 800 A, JP 1-122 183 A or EP 259 888 A, in which a laser diode element is present in combination with a Peltier element. However, there are no high laser powers and thus also merely low thermal loads on the laser diode elements, and the Peltier elements in fact are merely used for temperature stabilizing purposes. In the known semi-conductor lasers, this is important because in case of laser diodes, the laser wave length depends substantially on the temperature of the semi-conductor chip, and in many instances even its heating is required so as to obtain the correct wave length. Besides, in these known devices, an evacuation of the module or its drying by means of desiccating substances are not mentioned.
With the Peltier elements, in most instances sufficient cooling of the laser crystals can be achieved without any problems, and it has been shown that a temperature difference of approximately 50xc2x0 C. or 70xc2x0 C. at the Peltier elements will suffice in most instances. For a particularly pronounced cooling or heat dissipation from the laser crystal it may also be advantageous if the Peltier elements are provided in stacked manner. In this instance, temperatures of xe2x88x9250xc2x0 C. or xe2x88x92100xc2x0 C. may easily be reached on the cold side at an ambient temperature (approximately 20xc2x0 C.) on the warm side. As such, temperature differences at the Peltier elements of up to 130xc2x0 C., when using conventional Peltier elements, are possible so that cooling may be effected to temperatures of below xe2x88x92100xc2x0 C.
The optical crystal, or laser crystal, respectively may be platelet-shaped andxe2x80x94with a view to the good cooling attainablexe2x80x94of comparatively small dimensions, and also if used with an amplifier, its dimensions may be merely approximately 3 mm in width and length, with a height of merely 1 to 1.5 mm.
To fix the crystal while ensuring a good thermal transition and a good thermal dissipation, it is also advantageous if the crystal is held between cooling jaws of good thermal conductivity, against which the Peltier elements rest. In doing so, for attaining as large a thermal transition surface as possible as well as a particularly simple retention of the crystal it is, moreover, suitable if the cooling jaws positively embrace and retain the crystal at four sides thereof. A solution which is suitable in terms of production and mounting will moreover be achieved if one of two cooling jaws resting against the crystal at opposites sides thereof has a nose projection extending over the crystal resting on the other cooling jaw, and the cooling jaws are provided with recesses in front of or behind the crystal, respectively, in the direction of the laser beams for the laser beams to pass therethrough.
To keep the Peltier elements on the xe2x80x9cwarmxe2x80x9d side at ambient temperature (or even therebelow), it is furthermore advantageous if the Peltier elements are in engagement with a cooling pedestal on their warm side that faces away from the cooling jaws. For efficiently cooling the warm side of the Peltier elements it has also proven advantageous if the cooling pedestal is liquid-cooled. The cooling pedestal may have the most varying shapes, such as, e.g., cuboid or disk-shaped. To attain a high cold storage capacity as well as for a stable accommodation of the Peltier elements and the cooling jaws and for a simple production it is furthermore suitable if the cooling pedestal is formed by a generally cyllindrical body having a generally V-shaped recess at an end side which accommodates the Peltier elements as well as the cooling jaws with the crystal. For reasons of processing and also for the abutment surface of the Peltier elements and the cooling jaws it is advantageous if the V-shaped recess comprises an apex angle of 90xc2x0. To orient the Peltier elements and to facilitate their mounting it is, moreover, advantageous if the generally V-shaped recess defines oblique resting surfaces for the Peltier elements and stops for the Peltier elements are provided at the inner, adjacent ends of the resting surfaces, which stops project upwardly from the resting surfaces.
A particularly simple design of the encapsulated type container which allows for a good sealing, e.g. by means of O-rings, may be obtained if the container comprises a tubular casing closed by a lid. In this connection it is, furthermore, advantageous if the cooling pedestal at its end side facing away from the Peltier elements is provided with a flange with which the tubular casing is tightly connected. It is also advantageous if the cooling pedestal is provided with bores for the passage of cooling liquid in the region of the flange.
For the laser beams to have a low power relative to the area unit of their cross-section, when passing through the window, (so that they will not cause burning in or destruction of the windows after short periods of operation), the laser beams should have as large a cross-section as possible at the site of the windows, i.e. they should be out of focus, which means that for the windows a certain distance should be kept (e.g. approximately 8 to 10 cm) to the crystalxe2x80x94where focussing occurs. To make this possible without enlarging the entire container, it is also suitable if the encasing container, preferably at oppositely arranged sides thereof, is provided with a (respective) projecting, tightly attached pipe socket which, at its outer end, is closed by the window for the passage of the laser beams.
The invention also relates to a laser arrangement comprising a cooling device as explained above.